The present invention relates to a composition suitable for finishing, preserving and cleaning lithographic printing plates which have been imagewise exposed and developed. The art of lithographic printing depends on the immiscibility of greasy ink and water, upon the preferential retention of a greasy ink by an image area of a printing plate and upon a similar retention of an aqueous dampening fluid by a non-image area. When a greasy ink is imprinted on a suitable surface and the entire surface is then moistened by an aqueous solution, the image area will repel the water and the non-image area will retain the water. Upon subsequent application of greasy ink, the image portion retains the ink and the moistened non-image area repels it. The ink on the image area is then transferred to the surface of the material on which the image is to be reproduced, such as paper, via an offset blanket, which provides better image quality. The most common type of lithographic plate to which the present invention is directed has a coating of a light sensitive substance that is adherent to an aluminum base sheet. Depending on the nature of the photosensitive coating employed, the plate may be positive or negative working. In both cases, the image area is oleophilic and the non-image areas are hydrophilic. Exposure is effected through a transparent mask, wherein the light sensitive layer, in the negative working case, hardens and becomes insoluble in a developinq solution. When the developer is applied to the plate, the non-image areas are removed. The remaining areas become the portions which attract greasy ink and are called the image areas. The surface underlying the areas from which the light sensitive coating have been removed are hydrophilic, do not attract greasy ink and are called the non-image areas.
It is known in the art that after repeated use of the plate and ageing of the surface, that the non-image areas are less able to repel ink and may tend to retain some of this ink. This is called scumming. Therefore, if the surface properties between the image and non-image areas are disturbed, for example, if the hydrophilic property of the non-image areas is deteriorated for some reason, inks are likely to adhere to such areas with deteriorated hydrophilicity and cause background stains. Such background stains are formed under a variety of conditions, for example where a lithographic printing plate is subjected to a burning-in treatment for the purpose of increasing length of run, or in the case where the surface of a plate is allowed to stand in the air without protecting it with a desensitizing gum. It is usual in the art that lithographic printing plates which are ready for printing must be subjected to such a protective finishing treatment before they are stored for prolonged periods of time. As a typical treating solution a gum arabic solution is very widely used. Dextrin and polyvinyl alcohol solutions are also known. U.S. Pat. No. 4,033,919 teaches a combination of polymers of acrylamide containing carboxy groups with acids as desensitizing agents for plates. After treatment with a desensitizing solution, printing plates are usually stored for some time. It has been found that the oleophilic character, i.e. ink receptivity of the image areas of the plate is often considerably reduced upon storage, so that a large amount of paper is wasted on roll-up. Of course, in prolonged storage of desensitized plates, undesired reactions may also occur between the desensitizing material and the surface of the support, and as a result, the hydrophilic properties of the plate are impaired. Therefore, a good finisher/preserver must function to desensitize the non-image areas to assure that they will not accept greasy ink upon printing, and prevent blinding in the image areas. It must also prevent oxidation of the background areas of the plate during storage or while waiting for press mounting. It must also be quickly removable from the plate so that it will not cause production delays. Typically a finisher must be quickly removable by a water rinse or most preferably must be removable by the fountain solution on the press. Quick roll-up is then essential in order to prevent paper waste and reduced production time.
When a lithographic printing plate has become contaminated, such contaminated areas are rendered oil sensitive and result in background stains. The appearance of fingerprints in the background of prints is also ascribed to this condition. In addition, when the non-image areas take scratches, the scratches are filled with ink and are rendered oil sensitive and cause stains. In these cases, it is usual that the printing ink is first removed from the plate and then the stained areas are treated with a plate cleaner to restore the hydrophilic property of the non-image areas. Cleansing agents usually consist of dispersions and contain a number of heterogeneous substances such as those described in U.S. Pat. No. 2,780,168. Various other plate cleaning compositions are known as taught by U.S. Pat. Nos. 3,108,535; 3,289,577; 3,060,848; 4,162,920; 2,780,186; 3,679,479; and 3,489,561. In general, when stains are generated during the printing process, the surface is first treated with a hydrocarbon solvent to remove the ink and then with a desensitizing agent. This means in many cases that two steps are required.
Compositions for finishing, cleaning and preserving lithographic printing plates are well known in the art as exemplified by U.S. Pat. No. 4,162,920. Such are generally composed of an emulsion of an aqueous phase and a solvent phase. Principally the solvent phase dissolves the greasy inks built up on the plate, and the aqueous phase deposits on the image and non-image areas to protect them from atmospheric attack and to restore hydrophilicity to the background areas. It is important to produce an emulsion that is stable, i.e. the aqueous and solvent phases do not readily separate out. In this regard it is known to use gum arabic and dextrins, such as tapioca dextrin in the aqueous phase. Dextrins are obtained through either acid or alkaline hydrolysis of starches. Such dextrins are typically HCl hydrolyzed tapioca starches. It has now been unexpectedly found that when a finisher, preserver, cleaner emulsion is formulated with a maltodextrin prepared by enzyme hydrolysis of corn or potato starch, that emulsion stability is increased dramatically.